I have not been following your thread lately but I wanted to share something being as you are talking HVAC.
I am building a recording studio in Mpls and had the HVAC folks install sound attenuators. They are a little pricey but they are wonderful. They are about 2 feet long. I had one guy yelling in one side of it and I could not hear him from the other side. They work great and they just install in series with standard ductwork.
Didn't know if you...